Variable index material for optical switching and real time holographic recording

ABSTRACT

A new variable index material is described which provides rapid changes in refractive index. The disclosed materials are useful for coatings and films that provide photosensitive materials for holographic recording with high efficiency. The materials are also useful for modulating the coupling ratio in fiber optic couplers for optical switching. The new materials incorporate polymeric dialkylsilane ferrocenylene and long chain fatty acids.

BACKGROUND OF THE INVENTION

The U.S. government owns rights in the present invention pursuant togrant No. RRII-0880-2973 awarded by the National Science Foundation.

FIELD OF THE INVENTION

The present invention relates generally to high efficiency variableindex materials and, in particular, to materials used as specialcoatings on fiber optic couplers and as special emulsions for real timeholographic recording. The rapidly changing indices of refraction ofthese materials permits the modulation of the coupling ratio of fiberoptic couplers for optical switching. This property also contributes tothe high efficiency of photosensitive compositions prepared for use inholographic recording.

DESCRIPTION OF THE RELATED ART

Reversible variable index materials with fast response are organicsemiconductors involving a donor-acceptor mechanism. One of the firstmaterials studied was poly(N-vinylcarbazole) doped with the acceptortrinitro-2,4,7-fluorenone-9 (TNF). Higher changes in refraction indicesare now obtained with organometallic charge transfer complexes. Twoclasses of such complexes are complexes where (i) the metal is anintegral part of the backbone of the molecule and (ii) the metal is partof a lateral group.

Examples of the first category are shown in FIG. 1A, FIG. 1B, FIG. 1C,FIG. 1D and FIG. 2. In metal phthalocyanines such as the one representedin FIG. 2, charge transfer takes place between the molecular groupsthrough the ligands and the metal atoms. Electron transfer is improvedif the ligand is conjugated and the chelate structure is planar.

In the second class of variable index organic semiconductors, the metalmay be in the form of an ion, as shown in FIG. 3 and FIG. 4; a πcomplex, as shown in FIG. 5 and FIG. 6; or bound to a chelating ligand,as shown in FIG. 7. Polymers with chelating lateral groups easily formcoordination complexes with metal ions. Depending on the ionic chargeand the coordination number of the metal ion, crosslinking may occur.This is the case for compounds such as polyvinylferrocene where themetal induced crosslinking is facilitated by the cyclopentadiene groups(See FIG. 8). In such complexes the hybrid orbitals of the metal atomsreadily lose electrons and contribute to the donor part of the molecule.The electrons can be trapped at an impurity or a structural defect whichadditionally contributes to the index of refraction.

Conduction in an organic semiconductor depends on localized states inthe conduction band of the material (Halperin, 1967; Gutmann and Lyons,1967). These states can extend into the band gap as shown in FIG. 9A andFIG. 9B. The lowest possible states are localized states but there is acritical energy E_(c) above which the states do not become morelocalized. An electron can be promoted to a localized state in theconduction band either thermally, by photoelectric effect or byinjection. Once in that state it is necessary to provide sufficientenergy to displace the electron to another localized state. This energycan be thermal or photonic. The electron can also be transferred by atunnel effect if the orbitals overlap. The localized states correspondto traps for electrons generated photolytically in the material, asrepresented in FIG. 9A and FIG. 9B.

A trapped electron can oscillate under the influence of a periodicfield. The system trap-electron becomes an elementary dipolecontributing to the dielectric constant and the index of refraction ofthe material. This contribution is proportional to the maximumdisplacement of the electron during the oscillation, which correspondsto the depth of the traps, E_(c), and also to the density of the traps.The origin of the electron may be a donor group of the polymer; forexample, the hybrid orbitals of the metal atom of a chelating group.

The change in index of refraction in an organic semiconductor can beevaluated from the electronic properties of the material (Robillard,1990). Two cases are considered depending on whether the change isproduced by free carriers or by trapped carriers.

In the case of free carriers, the relative change is equal to: ##EQU1##where: η: index of refraction

ε: permittivity

N: concentration of free electrons

m: elementary mass of the electron

e: elementary charge of the electron

ν: frequency of the light.

For trapped carriers the relative change is represented by:

    1/ηΔη≃2πα•ΔN

where:

N: density of the trapped carriers

α: electronic polarizability at optical frequency.

SUMMARY OF THE INVENTION

The present invention addresses problems inherent in the art byproviding novel variable index materials with improved properties ascompared to materials presently employed for analogous applications. Theimprovements include complete reversibility, faster response times,higher refraction index variations, and increased stability. Theinvention also includes coatings and films that generate variablerefractive index patterns in response to an identical pattern of lightdistribution. The invention also addresses the fabrication of films forthe recording of high efficiency holograms.

As used herein the term "radiation sensitive member" refers to acomponent that responds to light by the absorption of visible radiationto increase its refractive index within the bulk of the film. Theradiation sensitive materials are essentially transparent and exhibitvariations in refractive index in response to radiation.

The inventors have developed novel variable index materials thatincorporate polymeric disubstitued silicon or germanium ferrocenyleneand long chain fatty acids. The ferrocenylene polymers employed in thepractice of the present invention are polymers containing ferrocenegroups linked together by disubstituted silyl or germanyl groups, asshown in FIG. 10. Examples of such polymers are poly(dimethylsilaneferrocenylene), poly(diethylsilane ferrocenylene), poly(dibutylsilaneferrocenylene), poly(dihexysilane ferrocenylene),poly(methylphenylsilane ferrocenylene, the corresponding germaniumanalogs, and the like. Other substituents, such as branched chainalkyls, mixed alkyl and mixed aryl alkyl groups may also be used assubstituents.

The inventors have achieved high efficiency holographic recording withphotosensitive mono and multilayer films by combining a fatty acid witha substituted ferrocenylene as a mixed component and particularly withdialkylsilane ferrocenylene. The fatty acids are typically organicmonobasic acids having the general formula C_(n) H_(2n+1) COOH derivedfrom saturated aliphatic hydrocarbons, such as eicosanoic acid, stearicacid, behenic acid, lauric acid, and the like.

The films are typically prepared from poly(ferrocenyl disubstitutedsilane or germane)/fatty acid mixtures spread in monolayers,conveniently by spreading on a subwater phase of a Langmuir Blodgettthrow. For certain applications, such as holographic recording plates,each monolayer formed is removed and deposited on a transparentsubstrate to form a multilayer film.

A preferred embodiment of this invention is a radiation sensitive memberhaving a dialkylsilane ferrocenylene polymer and fatty acid present as anumber of Langmuir Blodgett layers. These layers may be prepared byconditioning the polymer and fatty acid composition into a film orientedby the Langmuir Blodgett method.

A number of films may need to be stacked to obtain maximum performance.It is preferred to have the number of Langmuir Blodgett layers between20 and 100, with 20 layers being particularly preferred. When the filmabsorbs radiation (450 nm for the dimethylsilane-ferrocenylene polymer)a change in the index of refraction is initiated by a charge transfermechanism. Films made from the dialkylsilane ferrocenylenepolymers/fatty acids have provided high efficiency holographic recorders(on the order of 92% or greater).

The disclosed variable index materials have several applications,including holographic recording for real time recording. For example,films incorporating ferrocenyl dialkylsilane provide highly efficientholographic films. Such films are readily formed by Langmuir Blodgettmethods, and then transferred to an appropriate transparent substrate.Multilayer films are the most efficient. The number of layers will varydepending on the particular disubstituted silane (or germanium) andfatty acid employed, although typically the number of layers will bebetween about 15 and about 40. The sensitivity and holographicefficiency of the recording materials is dependent on the transparencyand variation of refractive index of the composition.

Other uses of the disclosed films include optical switches. Suchswitches can be made of a fiber optic coupler with variable indexmaterial surrounding the coupling area, as illustrated in FIG. 11. Asufficient change in index of refraction induced by irradiation of thematerial causes the light signal in fiber 1 to switch to fiber 2 with aswitching time in the picosecond range.

In a particular embodiment of the invention, a fiber optical coupler iscomprised of two optical fibers coated with a radiation sensitivemember. The two optical fibers may be brought in close proximity to eachother so that the evanescent fields of their signals overlap and part ofthe signal in one fiber is transmitted to the other (Bergh et al.,1980).

Another aspect of the present invention is a process for switchingoptical signals by exposing a fiber optical coupler to light; andswitching the light signal from one fiber of the coupler to the other.The excitation by exposure to light provides the change of index ofrefraction necessary to transfer the optical signal from one fiber tothe other. The coupling ratio, that is, the amount of signal transferredfrom one fiber to the other, is dependent on the index of refraction ofthe material surrounding the coupling region. If this index varies underthe influence of light, it is then possible to switch the transmissionof a signal from one fiber to the other. The switching speed depends onthe response of the variable index material and, with the new materialsdisclosed in this invention, is on the order of a few picoseconds.

Yet another aspect of the invention encompasses a process for recordingphase holograms by coating a transparent substrate with the radiationsensitive compounds and holographically exposing the film to radiation.Combining dialkylsilane ferrocenyl polymers with fatty acids, forexample, provides a material for making holographic recording platesthat allow high efficiency holographic recording. A film of the polymeris oriented by the Langmuir Blodgett method and transferred to theholographic substrate.

The high efficiency recording of phase holograms (Bragg holograms) canbe made using local variations in refractive index alone, in the absenceof any concurrently introduced optical absorption. In visualobservation, under normal lighting, the recorded film is completelytransparent.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1A. Example of an organometallic complex where the copper metal isan integral part of the molecule.

FIG. 1B. Example of an organometallic complex where the metal is anintegral part of the molecule.

FIG. 1C. Example of an/organometallic complex where the tin metal is anintegral part of the molecule.

FIG. 1D. Example of an organometallic complex where the copper metal isan integral part of the molecule.

FIG. 2. Molecular structure of copper phthalocyanine.

FIG. 3. Polyvinyl complex with monovalent ion.

FIG. 4. Polyvinyl complex with divalent ion.

FIG. 5. Polyvinylferrocene.

FIG. 6. Polystyrene complex.

FIG. 7. Polyvinyl alcohol-Cu complex.

FIG. 8. Crosslinking in polyvinylferrocene.

FIG. 9A. Organic semiconductor energy diagram showing conductance bands.

FIG. 9B. Organic semiconductor energy diagram showing valence bands.

FIG. 10. Molecular structure of a disubstituted silane orgermanium-ferrocenylene polymer where R is alkyl or aryl.

FIG. 11. Fiber optic coupler. Shown at number 1 is fiber 1; shown atnumber 2 is fiber 2; shown at number 3 is LED; shown at number 4 is VIM;shown at number 5 is photodiode 2; shown at number 6 is photodiode 1;shown at number 7 is Diff. amplifier; shown at number 8 is meter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The variable index materials of the present invention are compounds ofthe general structure shown in FIG. 10. The compounds are formed frompolymerization of disubstituted silane or disubstituted germaneferrocenylene.

The photosensitive materials of the present invention are useful in thepreparation of holographic films for high efficiency and real timeholography. The optical properties of poly (ferrocenyl diatkylsilane),for example, allow the design of variable index materials with unusuallyhigh refractive index variations on the order of 10⁻² that are usefulfor real time holographic recording. While not wishing to be limiting interms of explanation, the inventors suspect that the high refractiveindex variations of the disclosed materials may be due to adonor-acceptor interaction in the molecule where the hybrid orbitals ofthe metal atoms (Fe and Si or Ge) easily lose electrons that contributeto the donor part of the molecule and the photoconduction of thematerial. These electrons may be trapped as the acceptor part of themolecule, as an impurity or as a structural defect such as anunsaturated monomer link, providing an additional contribution to thedielectric constant and the refractive index of the material. Theintroduction of various substituents in the monomer may determine theindex of refraction and the magnitude of its variation.

A holographic recording member or film is made of several layers of oneof the disclosed ferrocenyl polymers and a fatty acid formed by theLangmuir Blodgett method and deposited on a transparent substrate.Holograms recorded on such holographic recording members, for examplemembers prepared from dialkylferrocenyl polymers, have a diffractionefficiency over 99%.

The variable index materials of the present invention may also beemployed in optical switches. An optical switch may be made of a fiberoptic coupler with the variable index material surrounding the couplingarea, as shown in FIG. 11. A sufficient change in index of refractioninduced by irradiation of the material causes the light signal in fiber1 to switch to fiber 2 with a switching time in the picosecond range.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described.

The following examples are provided for purposes of clarification andshould not be considered as limiting. One skilled in the art wouldrecognize in light of the present disclosure that although the specifiedmaterials and conditions are important in practicing the invention,unspecified materials and conditions are not excluded as long as they donot prevent the benefits of the invention from being realized.

EXAMPLE 1

Synthesis of Poly(dimethylsilane ferrocenylene)

1,1'-dimethylsilylferrocenophane. A slurry of1,1'-dilithioferrocene-tetramethylethylene diamine (8 g, 0.025 mol) inhexane (20 ml) was added to a solution of dimethyldichlorosilane (3.1ml, 0.026 mol) in hexane (100 ml). The reaction mixture was stirredovernight. Hexane and excess silane were removed under vacuum. Thesolids were dissolved in hexane and the solution filtered to removeLiCl. The solvent was again removed under vacuum and the product wasextracted from the residue by vacuum sublimation (0.01 mm, 40° C.) toform orange platelets. The yield was 60% (mp 72.75° C., λ_(max) (hexane)478 mm (Fisher et al., 1979).

Poly(dimethylsilane ferrocenylene), poly(FCSiMe₂).1,1'-dimethylsilyl-ferrocenophane (1.0 g, 4.72 mmol) was sealed invacuuo in a pyrex tube (1 cm×6 cm) and heated at 125° C. for 6 hours.The reaction product, an amorphous glass solid, was dissolved in benzeneand precipitated from hexane. The precipitate was collected byfiltration and dried in vacuuo. This procedure yielded 0.65 g (65%). Thepolymer was characterized by ¹ H, ¹³ C, ²⁹ Si NMR and UV spectroscopy. Asimilar procedure was used for synthesis of polymers with the ethyl,n-butyl, n-hexyl, mixed phenyl/methyl and ferrocenyl groups (Nguyen etal., 1993).

EXAMPLE 2

Synthesis of Poly(dibutylsilane ferrocenylene)

(n-C₄ H₉)₂ SiCl₂ (3.4 g, 16 mmol) in hexane (10 ml) was stirredovernight with 1,1'-dilithioferrocene-tetramethylethylene diamine. Thereaction mixture was filtered and solvent was removed in vacuo. Thedistillation of the resulting liquid (138° C., 05 mm Hg) resulted in theisolation of a liquid fraction (1.78 g) that solidified slowly at roomtemperature. The solid contained a mixture of 1,1'-di-n-butylsilylferroceneophane (yield 35%) and a glassy solid dark red residue. Theresidue was washed twice with hexane, dissolved in benzene and thepolymeric product was precipitated from hexane. The polymeric productwas filtered and vacuum dried to yield 0.8 g (18%).

EXAMPLE 3

Ferrocenylene compounds with germanium in place of silicon weresynthesized. The following example is illustrative of the procedure.Other disubstituted germanes may be synthesized by a similar procedure.

Synthesis of Poly(diethyl)germanium Ferrocenylene

To a mixture of ferrocene (3.0 g, 16 mmole) and TMEDA (5 mL, 33 mmole)dissolved in hexane (60 mL) was slowly added a solution ofn-butyllithium in hexanes (22 mL of a 1.6 M solution). After about 2 hrsan orange precipitate formed and the system was left stirring for 12hrs. The precipitate was washed with hexanes until essentiallycolorless, and 50 mL of hexane added, cooled to -78° C. and the mixturewas stirred. To this stirred system was added Et₂ GeCl₂ (3.23 g, 16mmole in 5 mL hexane) over a 2 hr period. The stirred solution waslsowly raised to room temperature and stirred overnight. The reactionmixture was filtered and the solvent removed under vacuum to leavered-brown crystals. No attempt was made to isolate this material, whichwas then dissolved in 20 mL of toluene and heated in a sealed tube to105° C. for 10 days. The toluene was removed under vacuum, and thepolymeric residue was dissolved in THF (10 mL) and precipitated byaddition of methanol (300 mL) to form the polymer as a yellow solid.Anal for C₁₄ H₁₈ FeGe (Galbraith Laboratories): Calcd. %C, 53.17, %H,5.74. Found: %C, 52.82, %H, 5.81. NMR, C₆ D₆, ppm: ¹ H: 4,315, 4,146,1,30, 1,10. ¹³ C: 73.30, 72.83, 71.07, 9.875, 7.405.

EXAMPLE 4

Langmuir Blodgett Films

An 0.1 mM solution of poly(ferrocenyl dibutylsilane) and 0.17 mMeicosanoic acid in dichloromethane was spread on a subwater phase of aLangmuir Blodgett throw. The subphase contained 1.22×10⁻³ M cadmiumchloride. The Langmuir layer was formed at a constant surface pressureof 25 mN/m at 10 mm per min. It was then transferred onto a glasssubstrate that was previously treated with octadecyltrichlorosilane inchloroform. This operation was repeated 20 times thereby producing a20-layer film. The film was exposed to radiation at 450 nm. A variationof index of 2.6×10⁻² was measured on an ellipsometer (Gaertner L108B)for a nominal value of the refraction index of n=1.72. Diffractionefficiency of 92% with a spatial frequency of 3000 mm⁻¹ was obtained onthe reconstruction of a diffraction grating.

Measurement of the refractive index was made on an ellipsometer Gaertnertype L1043 with the substrate positioned in such a way that thedirection of dipping (orientation of the substrate perpendicular to theair-water interface during deposition) was parallel to the plane ofincidence in the ellipsometer.

Indices of n=1.15 for layers of poly(dimethylsilaneferrocenylene)-Eicosanoic acid with variations Δn=2×10⁻² and n=1.96 forlayers of poly(dibutylsilane ferrocenylene)-eicosanoic acid withvariations Δn=2×10⁻² were measured on 20 layer films. The measurementswere made at λ=632.8 (HeNe) and the variations were obtained with 50microsecond pulses at 450 nm.

Films made with other (poly) disubstituted silane ferrocenyl andpoly(disubstituted germanium ferrocenyl compounds were obtained in amanner similar to that described. The films had similar properties tothose of poly(dibutylsilane ferrocenylene)-eicosanoic acid.

While the compositions and methods of this invention have been describedin terms of preferred embodiments, it will be apparent to those of skillin the art that variations may be applied to the systems, compositions,methods and in the steps or in the sequence of steps of the methodsdescribed herein without departing from the concept, spirit and scope ofthe invention. For example, it will be apparent that certain agentswhich are chemically, compositionally and functionally related may besubstituted for the agents described herein where the same or similarresults may be achieved. All such similar substitutes and modificationsapparent to those skilled in the art are considered to be within thespirit, scope and concept of the invention as defined by the appendedclaims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

Bergh et al. (1980) Electron Lett., 16:260-261.

Fisher et al. (1979) J. Am. Chem. Soc., 101(22):6501.

Gutmann and Lyons (1967) Organic Semiconductors, Wiley, New York.

Halperin (1967) Advances in Chemical Physics, 13:123.

Nguyen et al. (1993) Chem. Mater., 5:1389-1394.

Robillard (1990) in Industrial Application of Holography, Robillard andCaulfield Ed., Oxford University Press, New York, 136.

What is claimed is:
 1. A radiation sensitive member comprising a fattyacid and a disubstituted silane ferrocenylene polymer or a disubstitutedgermane ferrocenylene polymer having the structure of FIG. 10 wherein R₁and R₂ are independently alkyl, aryl or ferrocenyl wherein said memberabsorbs visible radiation to increase refractive index of said member.2. The radiation sensitive member of claim 1 wherein the alkyl ismethyl, ethyl, propyl, isopropyl, n-butyl, n-hexyl, mixed phenyl/alkylor ferrocenyl groups.
 3. The radiation sensitive member of claim 1wherein the aryl is phenyl or mixed aryl/alkyl.
 4. The radiationsensitive member of claim 1 wherein the radiation is between 400 and 800nm.
 5. The radiation sensitive member of claim 1, wherein thedisubstituted silane ferrocenylene polymer is poly(dimethylsilaneferrocenylene), poly(diethylsilane ferrocenylene), poly(dibutylsilaneferrocenylene), or poly(dihexysilane ferrocenylene).
 6. The radiationsensitive member of claim 1 wherein the disubstituted germaneferrocenylene polymer is poly (dimethyl germane ferrocenylene), poly(diethyl germane ferrocenylene), poly (dibutyl germane ferrocenylene) orpoly (dihexyl germane ferrocenylene).
 7. The radiation sensitive memberof claim 1 wherein the fatty acid is a long chain fatty acid.
 8. Theradiation sensitive member of claim 1, wherein the fatty acid iseicosanoic, stearic, behenic, or lauric acid.
 9. The radiation sensitivemember of claim 1 wherein the fatty acid is eicosanoic acid.
 10. Theradiation sensitive member of claim 1, wherein the fatty acid is mixedwith the disubstituted silane or germane ferrocenylene polymer.
 11. Theradiation sensitive member of claim 1, wherein the fatty acid iscovalently bound to the disubstituted silane or germane ferrocenylenepolymer.
 12. The radiation sensitive member of claim 1, wherein thedisubstituted silane ferrocenylene polymer and the fatty acid compriseone to about 100 Langmuir Blodgett layers.
 13. The radiation sensitivemember of claim 12 wherein the number of said Langmuir Blodgett layersis between about 10 and about
 100. 14. The radiation sensitive member ofclaim 12 wherein the number of said Langmuir Blodgett layers is about20.
 15. A process for switching optical signals comprising the stepsof:(a) coating an optical fiber coupling region with a monolayer orseveral monolayer film prepared from a fatty acid and a disubstitutedsilane or germane ferrocenylene polymer having the structure of FIG. 10wherein R₁ and R₂ are independently alkyl, aryl or ferrocenyl; and (b)exposing the coated coupling region to visible light to producevariations in refractive index wherein optical signals are switched fromone to the other optical fiber.
 16. A process for recording phaseholograms comprising the steps of:(a) coating a transparent substratewith a monolayer or several monolayer film prepared from a fatty acidand a disubstituted silane or germane ferrocenylene polymer having thestructure of FIG. 10 wherein R₁ and R₂ are independently alkyl, aryl orferrocenyl; and (b) holographically exposing said member to visibleradiation to produce variations in refractive index in exposed areas ofthe member.
 17. A phase hologram obtainable by the process of claim 16.